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Cobra Fiber-Optic Positioner Upgrade

Friday, 01 March 2013

This technology could be used for applications requiring precise location of a small object
within a small circular area, such as in medical lasers.

A prime focus spectrometer (PFS),
along with corrective optics, will mount
in place of the secondary mirror of the
Subaru telescope on Mauna Kea,
Hawaii. This will allow simultaneous
observations of cosmologic targets. It
will enable large-scale galactic archeology
and dark energy surveys to help
unlock the secrets of the universe.

To perform these cosmologic surveys,
an array of 2,400 optical fibers needs to be
independently positioned within the 498-
mm-diameter focal plane of the PFS
instrument to collect light from galaxies
and stars for spectrographic analyses. To
allow for independent re-positioning of
the fibers, a very small positioner (7.7 mm
in diameter) is required. One hundred
percent coverage of the focal plane is also
required, so these small actuators need to
cover a patrol region of 9.5 mm in diameter.
To optimize the amount of light that
can be collected, the fibers need to be
placed within 5 micrometers of their
intended target (either a star or galaxy).

The Cobra Fiber Positioner was
designed to meet the size and accuracy
requirements stated above. Cobra is a
two-degrees-of-freedom mechanism that
can position an optical fiber in the focal
plane of the PFS instrument to a precision
of 5 micrometers. It is a theta-phi
style positioner containing two rotary
piezo tube motors with one offset from
the other, which enables the optic fibers
to be placed anywhere in a small circular
patrol region. The patrol region of the
actuator is such that the array of 2,400
positioners allows for full coverage of
the instrument focal plane by overlapping
the patrol areas.

A second-generation Cobra positioner
was designed based on lessons learned
from the original prototype built in
2009. Improvements were made to the
precision of the ceramic motor parts,
and hard stops were redesigned to minimize
friction and prevent jamming.
These changes resulted in reducing the
number of move iterations required to
position the optical fiber within 5
micrometers of its target. At the time of
this reporting, there are still many tests
to be performed that will validate system
level performance, but on an individual
level, the Cobra positioner demonstrates excellent performance and will enable
the PFS instrument to make unprecedented
measurements of the universe.

What is unique about the upgrades
made to the Cobra positioner is the
improved performance due to the
design changes in the hard stops and the
ceramic end caps of the motors. Other
changes were made to reduce the unit
cost of a Cobra positioner without affecting
the performance, since thousands of
these devices will have to be built for the
PFS instrument.

This work was done by Charles D. Fisher,
David F. Braun, and Joel V. Kaluzny of
Caltech for NASA’s Jet Propulsion Laboratory.
For more information, contact iaoffice@
jpl.nasa.gov. NPO-48751

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